Relationship among IgE-binding factors with various molecular weights.

The rat-mouse T cell hybridoma 23B6 forms IgE-binding factors on incubation with IgE. The hybridoma cells incubated with IgE contained intracellular IgE-binding factors of 60K, 30K, 14K, and 10K daltons, and secreted the 60K, 30K, and 14K IgE-binding factors. Both intracellular and extracellular IgE-binding factors of 60K and 14K daltons selectively suppressed the IgE response, whereas the 30K and 10K factors failed to do so. Reduction and alkylation of the extracellular 60K IgE-binding factors yielded fragments of 30K, 14K, and 10K daltons with IgE-binding activity, and the same treatment of the 30K "inactive" IgE-binding factor yielded the 14K and 10K fragments. Among the fragments obtained by reduction and alkylation, only the 14K IgE-binding factors selectively suppressed the IgE response. Monoclonal antibody OX 3, which recognizes an Ia determinant, bound the 60K and 30K IgE-binding factor, and the 30K fragment obtained by reduction and alkylation. None of the 14K and 10K IgE-binding factors and fragments of comparable size obtained by reduction and alkylation contained the antigenic determinant. The results indicate that 30K, 14K, and 10K IgE-binding factors are derived from the 60K precursor molecules. Transfection of COS 7 monkey kidney cells with a single cloned cDNA, 8.3, which encodes rodent IgE-binding factor, resulted in the formation of IgE-binding factors of 60K and 11K daltons. Both species of the factors had affinity for lentil lectin and selectively potentiated the IgE response, but only the 60K IgE-binding factor bound to the monoclonal anti-Ia antibody. Reduction and alkylation of the 60K IgE-binding factors from the cDNA clone yielded an 11K fragment having the same properties as the 11K IgE-binding factor, with respect to the affinity for lentil lectin, and the biologic activities. Because the cDNA clone was constructed from mRNA of 23B6 cells, it appears that the 60K IgE-binding factor molecule from 23B6 cells is composed of a single peptide chain, and that the naturally formed 30K, 14K, and 10K IgE-binding factors are formed by post-translational modification of the 60K peptide.